Various medical devices are designed for surgical implantation into humans or animals. One common example is the cardiac pacemaker, which recognizes various dysrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the dysrhythmias. Another is the implantable cardioverter-defibrillator (“ICD”), which additionally (or alternatively) recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate the fibrillation. Pacemakers, ICDs and related devices for stimulating heart tissue are generally referred to as implantable cardiac stimulation devices. Other types of implantable medical devices include non-cardiac stimulation devices for stimulating or sensing portions of the brain, spinal cord, muscles, bones, nerves, glands or other body organs or tissues.
Within the implantable medical devices, particularly pacemakers and ICDs, it is often desirable to be able to detect the degree of activity of the patient in which the device is implanted, i.e. to determine whether the patient is standing, walking briskly, running, or walking up stairs. For example, within patients whose cardiac system is incapable of responding to an increase in physical activity, it is desirable to control the pacemaker to elevate the pacing rate while the patient is engaged in any potentially strenuous physical activity. Pacemakers that are responsive to the degree of activity of the patient are generally referred to as rate-responsive pacemakers. One technique for detecting the degree of activity of the patient is to employ one or more accelerometers that are responsive to the movement of the patient. A discussion of rate-responsive pacemakers and accelerometers for use therein is provided in U.S. Pat. No. 6,002,963 to Mouchawar et al., entitled “Multi-Axial Accelerometer-Based Sensor For An Implantable Medical Device And Method Of Measuring Motion Measurements Therefor”, Dec. 14, 1999, which is incorporated by reference herein in its entirety.
However, for some patients and under some circumstances, merely detecting the degree of activity the patient is not sufficient. For example, it may be desirable to provide a higher pacing rate while standing still than while sitting still even though the degree of activity is the same. However, a conventional activity sensor, which merely detects the degree of motion the patient, is not typically capable of distinguishing between the standing still and sitting still. Accordingly, it is also desirable to be able to detect the current posture of the patient, i.e. whether the patient is lying down, sitting or standing. One technique for detecting the current posture the patient is to employ a multi-axial accelerometer, of the type described in the patent cited above, which is capable of determining the orientation of the patient and is, in particular, capable of distinguishing between a prone patient and a standing patient based on the relative orientation of the accelerometer. However, such techniques are not entirely reliable when distinguishing between a seated posture and a standing posture, since the torso of the patient is typically oriented vertically in both cases. Accordingly, the device may erroneously determine that the patient is standing, rather than sitting, and provide an unnecessarily high pacing rate or, worse, the device may erroneously determine that the patient is sitting, rather than standing, and thereby providing a pacing rate that is too low, resulting in possible dizziness or feinting of the patient.
Accordingly, it would be desirable to provide improved techniques for reliably detecting the posture of the patient using an implantable medical device and it is to this and that aspects of invention are directed.
Moreover, in many circumstances, merely detecting in the posture of the patient is insufficient. Rather, it is preferable to detect a change in posture of the patient and to provide a temporary increase in heart rate when the patient changes posture in such a way as to require higher heart output. For example, it is desirable to temporarily increase the heart rate just as the patient rises from a seated position to a standing position to prevent dizziness or fainting, which might otherwise occur as a result of the inability of the heart of the patient to properly respond to the change in posture. Hence, devices that merely detect the current posture of the patient, even if achieved reliably, do not provide optimal functionality.
One possible technique for detecting a change in posture of the patient (which is not necessarily prior art to the present invention) is to detect the current posture following every burst of activity of the patient and then to compare the current posture with the last-known posture to thereby detect a change in posture. For example, if after a burst of activity, the device detects that the patient is now standing and the device had previously determined that the patient was sitting, and then the device can then conclude that the patient has stood up from a seated position and can increase the heart rate temporarily to prevent dizziness. However, for this technique to work reliably, the current posture and the previous posture must both be reliably and independently detected. Any error in the detection of either the current posture or the previous posture results in an erroneous detection of the change in posture, which, in turn, can result a pacing rate which is either too high or too low. As has already been mentioned, it can be difficult using conventional techniques to reliably detect the current posture and, in particular, to distinguish between sitting in standing. Accordingly, it is even more difficult to reliably identify a change in posture. In particular, since it is difficult for conventional techniques to distinguish between the seated posture and the standing posture, it is quite difficult to reliably detect when the patient rises from a seated position, again resulting in potentially adverse consequences if the proper pacing rate is not provided.
Accordingly, it would also be desirable to provide improved techniques for reliably detecting a change in posture of the patient using an implantable medical device and, in particular, for reliably detecting change in posture without a prior knowledge of the last posture and it is to this end that further aspects of invention are directed.
Additionally, once provided with a more reliable technique for detecting the current posture or change in posture of the patient, it would be desirable to provide enhanced medical device functionality that exploits the determination and it is to this end that still further aspects of invention are directed. For example, within ICDs, which deliver cardioversion shocks upon detection of atrial fibrillation, it would be desirable to selectively inhibit the delivery of such shocks based on the current posture or change in posture of the patient to better ensure the safety of the patient. In particular, it would be desirable to provide a technique for inhibiting the delivery of cardioversion shocks if the patient is found to be ascending or descending a staircase, as the delivery of such a shock at that time could result in serious injury.
Although the aforementioned concerns have been described primarily with reference to pacemakers and ICDs, similar or related concerns arise with respect to any implantable medical device requiring reliable detection of the current posture or change in posture of the patient and so it is desirable to provide improved techniques which can be exploited within a wide range of implantable medical devices.
Finally, given the processing, memory, and power of limitations inherent within implantable medical devices, it is desirable to provide techniques for permitting the aforementioned improvements to be most efficiently exploited within the implantable medical device, and still other aspects of invention are directed to that end. In particular, it would be desirable to provide set-up techniques for pre-calculating within an external programmer much of the information that the implantable medical device might require for posture determination such that the information may then be transmitted to the implantable device for use therein to reduce the resources required within the actual implantable device. Preferably, the pre-calculated information is specific to the particular patient. Still other aspects of the invention are directed to these ends.